There has been interest in recent years in photodetectors that have response times of approximately several tens of picoseconds or less. This interest in high speed photodetectors has been stimulated by several developments. For example, communications systems using glass fibers, commonly termed "optical fibers", to optically couple light sources and photodetectors have developed to the point where gigabit data rates are presently contemplated. The photodetectors used in such system need to have picosecond response times. High speed photodetectors are also increasingly desired and necessary for the accurate measurement and characterization of, for example, fast opto-electronic processes such as the dynamic response of semiconductor lasers.
As might be expected, several approaches have been taken in attempts to develop high speed photodetectors. One such approach uses GaAs metal-semiconductor field effect transistors, commonly referred to MESFETs, as high speed photodetectors. Such MESFETs were initially developed for discrete microwave devices and for high speed logic circuits. The high speed optical response of a GaAs MESFET was reported by Baack et al in Electronics Letters, 13, p. 193, 1977. Additional high speed GaAs MESFET devices were reported in Japanese Journal of Applied Physics, 19, pp. 27-L29, January 1980, and by Gammel et al in IEDM Technical Digest, p. 120, 1978. Devices using this approach are attractive candidates for high speed photodetectors because of their rapid response time and the possibility they afford of achieving optical gain.
However, the MESFET photodetectors reported in the literature generally suffer drawbacks that limit their usefulness as detectors in optical communications systems. For example, they typically have a thin active layer, approximately 0.2 .mu.m, and consequently, the incident radiation may be absorbed primarily in the semi-insulating substrate which is often filled with traps. Furthermore, the photodetectors generally have a small gate-drain spacing, typically approximately 2 .mu.m, which results in poor optical coupling efficiency between the incident radiation from, for example, an optical fiber, and the photodetector or requires precise alignment of optical beam and the detector.